In the injection moulding of a thermoplastic material, this material is injected in a warm, soft state and under pressure through an injection nozzle into the mould cavity in an injection mould. In this process, at the outflow aperture of the nozzle, where the material enters into the mould cavity, there has to be an abrupt transition from the temperature of the channel running through the nozzle to the temperature of the mould cavity, in order to ensure that when the finished article is being removed from the mould, a clean break is obtained between this article and the material still present in the outflow aperture. However, such an abrupt temperature transition cannot be achieved since, due to the fact that in injection moulding very high pressures are used, and the mould is made of a high-grade metal and/or ceramic material, a certain heat transfer will always take place between the nozzle and the mould.
For the solution to this problem, in the device according to the above-mentioned publication, use is made of a so-called shut-off pin which in combination with the above-mentioned seat at the outflow aperture can shut off said aperture mechanically, so that after the injection moulding of an article, by taking the shut-off pin into the shutting-off position, the material in the feed channel is separated from the material injected into the mould cavity, as a result of which a clean break is obtained.
In the known nozzle the pin is guided in sealing fashion through a hole in a sealing bush which is disposed above the above-mentioned one end of the channel running through the nozzle, at which end said channel is connected to a feed of the material. This feed passes here into a bend or loop. In this way the pin is thus conveyed to an area which does not contain any material when the nozzle is in operation.
The disadvantage of such a sealing bush is that a permanently good sealing engagement of the pin is difficult to achieve, since account has to be taken here of the fact that very high pressures are used for injection moulding, many plastics are chemically very active at high temperature, and the plastics used can contain abrasive fillers such as glassfibre and chalk, while the shut-off pin is often moved quickly to and fro millions of times. Due to the high pressures used, the sealing bush has to be of a very strong, rigid material which cannot easily be deformed, so that a self-correcting seal round the pin is virtually impossible to achieve.
The object of the invention is to produce an injection nozzle of the above-mentioned type which does not have this disadvantage.